Thermal dehydration in humans is associated with a decrease in work capacity and is a major contributing factor to heat stroke. Thirst induced by thermal dehydration is important in preventing these detrimental effects of thermal dehydration. The objective of the proposed research is to determine the physiological pathways involved in the thirst observed following thermal dehydration using rats as a model system. Specific aims of the proposed research are to a) determine the relationship between water intake and the degree of thermal dehydration and to determine the roles of b) increased osmolality, c) hypovolemia, and d) increased body temperature in thermal dehydration-induced thirst as well as the roles of the e) reninangiotensin system, f) adrenergic system, g) cholinergic system and h) opiates in this type of thirst. Male rats will be used as the experimental animal in these studies. Rats will be exposed to a hot environment (30-45 degree C) without water for up to 5 hours to induce thermal dehydration. Water intake following removal from the heat will be correlated with body temperatures and measurements of the degree of dehydration such as body weight changes, hematocrit, plasma protein and sodium concentrations, plasma osmolality and plasma renin activity. Selective replacement of plasma volume with saline administration or returning osmolality to normal with water infusion will be used to determine the roles of hypovolemia and increased osmolality in thermal dehydration-induced thirst. Experimental manipulations of colonic and skin temperature will be used to study the role of body temperatures in this type of thirst. Central and peripheral pharmacologic blockade of the reninangiotensin system, adrenergic, cholinergic and opiate receptors will be used to study the roles of these systems in thermal dehydration-induced thirst Information from these studies will be of direct importance in aiding our understanding of heat- induced thirst and should aid in designing interventions to prevent thermal dehydration and its health consequences.